It is known that the volume of liquid flowing from a vessel having a fluctuating liquid level therein through an opening in the vicinity of the base of the vessel is not constant over a period of time, but varies proportionally to the square root of the height of the liquid level in the vessel. In many cases it is desirable to maintain a continuous liquid discharge from a vessel or tank that is constant over a period of time, regardless of the varying height of the liquid level. Examples of this are storm water retention tanks, located ahead of waste water treatment plants and adapted to store quantities of waste water which increase considerably under storm weather conditions and thus exceed the capacity of the treatment plant. Depending on the volume of rainfall, the liquid level in such storm water retention tanks can fluctuate greatly and at the same time, the liquid from the tank is to be discharged at a level as constant as possible into the treatment plant, according to the maximum capacity of the plant.
The discharge of liquid at a constant level over a time period from the vicinity of the surface of the liquid in a tank can be effected by a floating inlet following the fluctuations in the liquid level. However, in the case of storm water retention tanks and in many other cases it is necessary to locate the liquid discharge as low as possible in the vicinity of the base of the tank. For such cases it is known that valve control devices such a butterfly valves, butterfly flaps, pneumatic valves or the like, can be located in the discharge pipe. These are controlled by floats or other control members, depending on the liquid level in the tank, and provide a constant discharge quantity over a time period by relative variation of the flow cross section of the discharge pipe.
Difficulties have been encountered with these devices due to the fact that the control is relatively inaccurate as it is difficult to achieve a variation of the flow cross section, proportional to the square root of the liquid height in the tank. Also, the fluctuating friction and vortex losses in the pipe and in the butterfly valve control device are very difficult to calculate. In addition, butterfly valve control devices are subject to wear and blockage due to pollutants in the waste water. Furthermore, such prior art devices are often very expensive, especially if precise electrically controlled butterfly units are used.